最小錯乱円

disk of least confusion

電子レンズには球面収差があるので、物体の光軸上の一点からいろいろな方向に出射した電子線は理想像面(収差がないとしたときの結像面(ガウス像面))で一点に集まらない。光軸となす角が小さな出射ビームは理想像面に近いところに像を作り、光軸となす角が大きいビームは理想像面よりレンズに近い側に像を作る。これらを加え合わせると、理想像面から少しレンズ側にずれたところに最小の円(ディスク)像が作られる。これを最小錯乱円という。球面収差による最小錯乱円の直径d_sは d_s = (1/2)Csα³で与えられ、ガウス面上でのボケの1/4である。ここでCsは球面収差係数、αは電子線の光軸とのなす角。

Since the electron lens has spherical aberration, electron beams, which exit from a point at the optical axis on the object plane while traveling in various directions, do not come to one point at the optical axis on the ideal image plane (Gaussian plane). An exiting beam, whose angle to the optical axis is small, nearly comes to the optical axis on the ideal image plane. An exiting beam, whose angle with respect to the optical axis is large, intersects the optical axis above the ideal image plane, thus deviates from the optical axis on the ideal image plane. Adding these formed images produces a least circle (disk) image at a position shifted a little from the ideal image plane to the objective lens. This circle is called "disk of least confusion." The diameter of the disk of least confusion, d_s, is given by d_s = (1/2)Csα³, which is 1/4 of blur on the Gaussian plane. Here, Cs is spherical aberration coefficient, α is the angle between the electron beam and the optical axis.